[5.04] Ultraviolet Spectroscopy of Near-Infrared--Selected QSOs

The Two-Micron All Sky Survey (2MASS) has found a large
population of previously unknown Active Galactic Nuclei
(AGN). These objects more than double the space density of
AGN at low redshift (z < 0.6) and were found from their
``red'' near-IR colors (J - Ks > 2). This new AGN sample
includes many objects with near-IR luminosities comparable
to those of traditional ultraviolet-excess QSOs. Luminous
2MASS AGN (objects having MK_s < -23) have a median
MK_s = -26.6 that is nearly identical to the
Palomar-Green QSOs (median MK_s = -26.7). Despite this
similarity, the optical properties of 2MASS and UV-excess
QSOs are vastly different with the near-IR--selected objects
being optically much fainter. The median B - Ks\/ color
index of the 2MASS QSOs is nearly two magnitudes redder than
that of PG QSOs in the same redshift range. In addition,
more than half of the 2MASS sample is composed of
narrow-line (Type~2) AGN and objects of intermediate optical
spectral type (Type~1.5--1.9). Together with higher levels
of optical polarization observed for 2MASS QSOs, these
properties suggest that 2MASS has revealed a large number of
AGN that are partially obscured by dust associated with the
nucleus. Also, their X-ray faintness relative to other AGN
indicates N\rm H ~1021--1023~cm-2. We
present preliminary results from UV spectroscopy of a small
sample of X-ray--observed 2MASS QSOs obtained with the
Space Telescope Imaging Spectrograph\/ (STIS\/).
2MASS QSOs, if detected in the UV by STIS\/, all show
very steep UV spectra and are extremely faint in the
ultraviolet compared to PG QSOs of similar near-IR
luminosity. The Hubble Space Telescope\/ observations
and contemporaneous ground-based optical spectroscopy
obtained to date are consistent with dust extinction and
reddening of an otherwise typical AGN spectrum. Simple
models like the unified scheme for AGN adequately explain
the range in infrared, optical polarization, and X-ray/UV
absorption properties of the 2MASS QSOs. Support for this
work was provided by NASA through grant number GO-09161.01
from the Space Telescope Science Institute, which is
operated by Association of Universities for Research in
Astronomy Incorporated, under NASA contract NAS5-26555.
2MASS is a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Center/California
Institute of Technology, funded by NASA and NSF.

The author(s) of this abstract have provided an email address
for comments about the abstract:
psmith@as.arizona.edu